Gas turbine engines employ sealing devices in various capacities where it is necessary to restrict the flow of fluid or gases from one portion of the engine to another. A common use is for separating the primary engine flowpath from the secondary flowpath. The primary engine flowpath directs the flow of gases to the compressor and turbine stages, from which the engine derives thrust or power. The secondary flowpath comprises a series of conduits for delivering compressed air throughout the engine for performing a variety of functions. Compressed air is used, for example, to cool individual components, provide a bleed air source, buffer the lubricated bearing cavities, control the ventilation among engine cavities and structures, and affect the thrust balance of the engine. Loss of compressed air from the secondary flowpath through leakage can have a substantial adverse effect on the performance of these functions. In a turbine engine, at least one sealing device typically is required for each turbine and compressor stage of the engine.
Another common use for sealing devices in turbine engines is for separating the secondary flowpath from engine cavities containing fluids such as lubricating oil. In pressurized aircraft, bleed air taken from the secondary flowpath supplies the aircraft environmental control system. Even small amounts of oil in the bleed air can render it unsuitable for this purpose. Further, oil leakage can lead to coking of the seal, and ultimately reduced seal life. To prevent this, buffered sealing devices typically are incorporated adjacent lubricated bearings and engine oil sumps.
Among the sealing devices more recently developed is the finger seal. Finger seals are comprised generally of a plurality of flexible members fixed at one end, the opposite ends sealingly engaging a surface that is rotatable relative thereto. The fingers of a finger seal are an integral part of the seal, usually formed by machining a series of curved slots in a forged ring or a length of sheet stock, the slots being of consistent length and extending from a common edge of the material. A complete seal is usually made up of two or more layers of fingers relatively positioned such that the gaps between fingers in one layer are blocked by the fingers of the next layer.
During engine operation, the fingers deform in a generally radially outward direction due to various factors including centrifugal growth of the rotating surface. In order to maintain an effective seal, it is also necessary for the fingers to restore themselves radially inward as the rotating surface shrinks. Fluid pressure acting on the radially deformed fingers, however, causes the fingers to deform axially often resulting in forceful contact between the fingers and adjacent structures. This axial deformation creates a frictional force that hampers radially inward recovery of the fingers as rotational velocity of the rotatable surface subsides. Consequently, the fingers are unable to re-engage the movable surface as it shrinks. Accordingly, the desired seal therebetween is undermined leading to engine inefficiencies.
Accordingly, a need exists for a seal capable of preventing or reducing the frictional forces between the finger members and adjacent structures such that the seal between the finger members and the movable surface is maintained during the full range of engine operation speed and power levels.